1. The field of the invention
Proportioning devices for vehicle hydraulic brake systems.
2. Description of the Prior Art
It is well known that the proportion of a vehicle's weight that is borne by the front and rear wheels of the vehicle changes as the vehicle is braked. In particular, during the braking operation, the weight borne by the rear wheels of a vehicle decreases while the weight borne by the front wheels increases. The change in weight is dependent upon the magnitude of the deceleration of the vehicle, and in recognition of this phenomena, various proportioning devices or valves have heretofore been proposed that are responsive to master cylinder outlet pressure and which serve to limit the brake pressure applied to the rear wheels upon attainment of a predetermined master cylinder outlet pressure. In the event of a front line failure or extreme brake fade at the rear wheels, it has also heretofore been proposed to bypass the proportioning device so that the brake pressure delivered to the brakes at the rear wheels is not in any way inhibited, thus assuring that the rear brakes have adequate line pressure to stop the vehicle.
In the design of vehicular brake systems, the front and rear brake systems are designed to reach a point of lock-up at some predetermined pressure ratio. In the case where a vehicle brake system incorporates disc-type brakes on the front wheels and drum brakes on the rear wheels, a typical "lock-up" point might be approximately 1200 PSI at the front brakes and approximately 800 PSI at the rear brakes. Due to the difference in fade characteristics between disc and drum type brakes (drum type brakes tending to fade at a considerably faster rate than disc brakes), under prolonged braking conditions which result in brake fade, the fluid pressure at which the rear brakes will lock up is considerably greater than the fluid pressure at which the front wheel disc brakes will lock up under the same braking conditions. Consequently, it is desirable that a decreasing offset or pressure differential between the pressures delivered to the front and rear brakes is established above the normal pressure at which the proportioning valve performs its proportioning or fluid modulation operation, whereby to achieve, at a predetermined rate, equalization of the fluid pressure delivered to the front and rear brakes. It is accordingly a general object of the present invention to provide a new, improved proportioning device which is adapted to be inserted between the master cylinder and a rear brake cylinder of a vehicular braking system for controlling the brake pressure applied to the rear wheels above a first predetermined master cylinder outlet pressure, and additional means is provided for controlling the pressure differential between the pressures delivered to the front and rear brakes above a second predetermined brake fluid inlet pressure, whereby to accommodate for the differential fade characteristics between the front and rear brakes of the vehicle.
It is another object of the present invention to provide a new and improved proportioning device, as above described, that may be operatively associated with a bypass valve arrangement which functions to communicate master cylinder pressure directly to the rear brake cylinders in the event of a failure of the front brakes of the associated vehicle.
It is another object of the present invention to provide a new and improved proportioning device, as above described, that is adapted to utilize readily available components, which is simple in operation and will have a long and effective operational life.
Other objects and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.